I was going by these benches, which I saw last year. They show triple channel being significantly faster than dual channel.
We left this testing session with the opinion that the triple channel configuration is a definite plus for the consumer and happens to be very logical progression in the ever increasing performance of memory.
Of course it is not going to be a night and day difference, but triple channel is faster, and 6GB of DDR3 should be cheaper than 8GB. I really see no reason to go with dual channel on that setup.